316 ABSORPTION OF NITROUS GASES
Dutt and Sen l point out that when nitric oxide is passed
through suspensions of various dioxides in water, mixtures of
nitrites and nitrates are formed. With lead dioxide (Pb02),
for example, a mixture of lead nitrate and nitrite is obtained.
Barium peroxide, on the other hand, gives exclusively barium
nitrite. The use of peroxides, however, in absorption practice
has not been made on any considerable scale up to the present
time, and it is difiicult to see that the use of such substances
could justify itself from an economic point of view.
Absorption in Solid Bases. The difficulties accompany-
ing the absorption of nitrous gases in towers by circulating
suspensions of basic substances, and the size of the absorption
units necessary, have given considerable impetus to the
examination of proposals for absorbing the gases directly in
a solid absorbent. Generally speaking, the proposals cover
the use of weakly basic oxides. Some of them are merely
processes for the concentration of the nitrous gases by the
formation of easily decomposable nitrites and nitrates (see
Chapter II). There are a number of others, however, which
cover processes yielding pure nitrates and nitrites. By far the
most important of these processes is that of Schloesing,2 who
proposes to use briquettes of lime at a temperature of 300°-
400° C., under which circumstances only calcium nitrate is
formed, the nitrite being unstable at that temperature. The
process is, so far as can be ascertained, the only method of
absorption by solid absorbents which has been tried on a
sufficiently large scale to justify its consideration as a com-
mercial possibility.
The presence of moisture or carbon dioxide in appreciable
quantity in the gases to be absorbed is undesirable, since the
former leads to a partial fusion of the absorbing material when
a certain amount of nitrate has been formed, while the latter
converts part of the lime into carbonate, which is comparatively
useless for the absorption. The lime employed must be pure
and uniform in size. It is also found that lime burnt at as
high a temperature as 900° C. is unsuitable for the absorption,
and the usual procedure is to slake such lime by the addition
of water, so that a homogeneous product is obtained, and then
1 Chem. Soc. Abstr., 1917, ii., 85.
2 Eng. Pat. 22,119, 1913.